(CNN) --The Russians and Americans may have ended their rivalry beyond Earth, but another contest for dominance in space remains, one that pits biology and brains against circuits and chips.

The loss of seven space shuttle astronauts this month again brought home the serious risk that humans face into the hostile environs beyond our sheltered planet, reviving the question of whether exploring the heavens should be left to unmanned missions.

Although less glamorous, such missions pose no risks to humans.

And already robots have an impressive list of accomplishments compared to the their flesh and blood counterparts.

"Unmanned missions are all about specifically targeted scientific explorations of the universe around us. Without exceptions, these efforts rise out of intense competition where the most compelling, and answerable, questions are posed," said Marc Buie, an astronomer at Lowell Observatory in Flagstaff, Arizona.

Robots have dug in the dirt on Mars, flown in the atmosphere of Jupiter, driven by the moons of Neptune and plopped down on an asteroid. A few are even flirting with the boundary of the solar system.

Humans, on the other hand, have been relegated mostly to going in circles, barely above the surface of the planet.

Besides the brief Apollo mission triumphs on the moon, almost 250,000 miles away, humans have never strayed farther than 400 miles from the planet, less than a day's drive, albeit straight up.

Humans fly near home for several reasons. There is the logistics of sending humans and what they need, like water, air and food, in a closed environment that keeps them alive. Robotic probes can travel with a fraction of the luggage, and therefore cost.

A typical shuttle mission, for example, runs between $400 million and $500 million. A satellite can reach orbit for $20 million.

Unmanned landers have touched down on Mars for as little as $250 million. But the estimated price tag for a manned journey to the red planet runs anywhere from $50 billion to $500 billion.

Muscle into mush

Second are the hazards: Sudden bursts of solar radiation can kill an unprotected spacewalker. Collisions with small space flotsam can obliterate a ship. And as the shuttle Challenger in 1986 and Columbia in 2003 painfully illustrate, launch and landing glitches can turn deadly in an instant.

Then there is the question of weightlessness, which over months can seriously weaken human bones, muscles and immune systems. And a roundtrip to Mars using current rocket power technology could take several years.

"Until we lick extended weightlessness, radiation and high-energy propulsion for us humans, [long-distance manned spaceflight] is a still a scientific dream. For the foreseeable future, robots offer our best scientific reality," said Wesley Ward, chief space geologist for the U.S. Geological Survey.

Critics of manned flight suggest an even more daunting challenge, lack of focus. To them, the flagship of NASA's current manned space program is drifting.

"The international space station, like the shuttle, is an instrument in search of a purpose," Ward said. "[We] are doing a variety of piddley experiments with little larger application to anything," he said.

The station, envisioned as an ambitious laboratory with a permanent crew of seven, has been plagued by delays and cost overruns. The eventual price tag, including shuttle deliveries of major additions, could reach between $60 billion and $100 billion.

For now, it hosts a crew of three, who have little time to devote to experiments. According to a National Research Council report, three astronauts can perform only 20 hours of science a week. The remainder of their time is spent keeping the station afloat.

Given the danger, the costs and the setbacks, why send humans into space at all? Some would argue the scientific value.

Cordless drills, bouncing bras

Drawing of the Cassini probe, which after a seven-year journey should arrive in the Saturn system in 2004.

For decades, NASA has gone to lengths defending the science of manned missions, presenting the billions spent on sending a few people into space as a worthwhile investment to the billions of people on the ground.

Cordless drills, fire detectors, CT scans and ionized water filters can trace their origin to the U.S. space program, according to NASA.

Critics contend that NASA has exaggerated the link between the space program and Earth-based technologies. Besides, they say, the spinoffs have come not from research in space but brainstorming on the ground; engineers could have thought up fire detectors without actually building rockets.

Nonetheless, many agree the most important scientific legacy of the manned space program, in particular the space station, is the astronauts themselves.

Dotted with sensors and given a battery of tests before, during and after months-long journeys, they provide a wealth of data about how to counteract the effects of weightlessness.

"There is widespread agreement, even among those otherwise critical of the station science, that it is an essential platform for research related to the impacts on humans of long duration spaceflight," said John Logsdon, director of the Space Policy Institute at George Washington University in Washington, D.C.

Logsdon contends reasons other than science justify sending people off the planet.

"We do not give parades for robots," he quipped. "There are intangible benefits from human presence in space -- national pride, role models for youth -- that are real if not measurable."

Parades are given to heroes, those who risk their lives for what they consider the higher good, whether for their nation or the world.

In his first public comments since the loss of Columbia, space station commander Ken Bowersox explained:

"The reason I come up here to space is because I believe ... that we're laying the foundation for our children, and their children to leave the planet someday."

Added crew mate Don Pettit: "You don't say, I will accept an increased risk so I can collect one particular piece of scientific data."

"You accept the risks for the sake of exploration, which is what's going to provide resources to use and places to live for our babies and grandbabies."

Universe or bust

Dale Gardner, an astronaut on the space shuttle Discovery in 1984, makes a lighthearted pitch after helping recapture a satellite.

So who should explore space? When grilled by Capitol Hill lawmakers last week, NASA administrator Sean O'Keefe expressed support for both man and machine.

"It's not a question of either or, robotics or humans," O'Keefe said. "The strategy we try to employ is not an either or but the best of both."

He cited shuttle missions to the Hubble Space Telescope as an example. Several times visiting astronauts, including from Columbia in 2002, performed major surgery on the orbiting observatory, each time improving its vision.

"It was considered a piece of space junk 10 years ago," O'Keefe said. "Human intervention was needed to fix the telescope."

Interestingly, robots might someday take over some spacewalking chores. NASA is working on a prototype called Robonaut to handle more mundane tasks of astronauts in space.

But Robonauts would supplement, not replace, the work of humans, whose depth and breadth of performance is beyond current robotics capability, according to Chris Culbert, a robotics researcher at NASA's Johnson Space Center in Houston, Texas.

Likewise, observatories might scan the heavens for distant planets. And probes might visit shrouded moons to find what lurks beneath. But for some nothing can replace the human experience beyond the confines of Earth.

"Manned missions are about expanding the boundaries of the human presence and experience. This comes from a deep-seated drive to see what's out there. This drive is what led early explorers to brave the oceans in frail little boats," Buie said.

"I personally believe that to keep thriving, the human race must go on expanding and exploring new frontiers. If we are doomed to remain on this planet, we will eventually die out."